1. Field of the Invention
The present invention relates to an image processor for performing color conversion on color data to output color data having a color space different from that of the former color data, and a color conversion method using the image processor.
2. Description of the Prior Art
Generally, color characteristics of peripheral equipment such as a scanner, a digital camera, a printer, a printing machine and a display depend on a manufacturer or a type of equipment. A color management system (CMS) is proposed and employed in order to use such peripheral equipment and to perform consistent color reproduction without dependence on color characteristics of the peripheral equipment.
FIG. 19 is a diagram showing a data flow in a conventional color management system CMSj.
In the color management system CMSj shown in FIG. 19, color data CD1 outputted from an input device DV1 are subjected to color conversion, lightness characteristics conversion and various other conversion and adjustment, so that data after the conversion/adjustment are outputted to an output device DV2 as color data CD2.
The input device DV1 may be, for example, a scanner, a digital camera, a display or others. The color data (image data) CD1 handled in the input device DV1 are usually RGB color system data (RGB data) that represent colors using three primary colors of R (red), G (green) and B (blue). The color data CD1 outputted from the input device DV1 are data depending on (or subordinating to) a color characteristic unique to the input device DV1.
The output device DV2 may be, for example, a printer or others. The color data (image data) CD2 handled in the output device DV2 are usually CMYK color system data (CMYK data) that represent colors using four colors (three primary colors plus black) of C (cyan), M (magenta), Y (yellow) and K (black). It is necessary to create the color data CD2 outputted to the output device DV2 so as to reflect a color characteristic unique to the output device DV2. In this regard, it can be also said that the color data CD2 are data depending on the output device DV2.
Thus, the input device DV1 and the output device DV2 differ from each other in a color system of color data CD to be handled. In addition, the color data CD handled in each of the input device DV1 and the output device DV2 depend on a color characteristic of the device, namely, the color data CD are device-dependent. Accordingly, in the color management system CMSj, color data are subjected to color conversion from one color system to the other color system and differences of color characteristics are adjusted.
In the color management system CMSj, for example, an XYZ color system and an L*a*b* color system are used as color systems independent of the input and output devices, i.e., device-independent color systems. Device profiles that are prepared for the respective devices are referred to for conversion between color data CD depending on each of the devices and color data CE of a color system independent of each of the devices.
More specifically, with respect to the input device DV1, an input profile PF1j that is a device profile on the input side is referred to in order to convert the color data CD1 of an RGB color system into color data CE1 of the XYZ color system. With respect to the output device DV2, an output profile PF2j that is a device profile on the output side is referred to in order to convert color data CE2 of the L*a*b* color system (hereinafter, referred to as the Lab color system) into the color data CD2 of a CMYK color system. Profiles conforming to International Color Consortium (ICC), i.e., ICC profiles are generally used as those device profiles.
The input profile PF1j includes, for example, a one-dimensional look-up table LUT1 and a matrix MX1. The one-dimensional look-up table LUT1 serves to correct a lightness characteristic of the input device DV1. The matrix MX1 is a group of coefficients for arithmetic expressions or operations used for converting the corrected color data CD1 into the color data CE1 of the XYZ color system.
The output profile PF2j includes, for example, a one-dimensional look-up table LUT2, a three-dimensional look-up table LUT3 and a one-dimensional look-up table LUT4. The one-dimensional look-up tables LUT2 and LUT4 serve to adjust variation in lightness of the output device DV2 and contrast of an image. The three-dimensional look-up table LUT3 is a look-up table used for converting the color data CE2 of the Lab color system into the color data CD2 of the CMYK color system.
A color conversion portion CC1j performs a color conversion process of converting the color data CE1 of the XYZ color system into the color data CE2 of the Lab color system. Further, the color conversion portion CC1j performs gamut mapping GM before or after the color conversion XL. In the gamut mapping GM, mapping by a predetermined conversion method is so carried out that the color data CD2 fall within a range where color reproduction is possible by the output device DV2 (within a color gamut). As the conversion method of the gamut mapping GM, “perceptual”, “preserving a relative color gamut”, “preserving an absolute color gamut” and “saturation” are known.
The color conversion in the color management system CMSj, which is described above, is actually carried out by a color management module. More specifically, the color management module (CMM) uses the input profile PF1j and the output profile PF2j and performs color matching between the input device DV1 and the output device DV2 via a device-independent color space. On this occasion, the color management module integrates steps of a color conversion process including the input profile PF1j, the color conversion portion CC1j and the output profile PF2j into one device link profile in advance and uses the device link profile to convert color data CD1 to be inputted into color data CD2 directly. Profiles defined by the ICC mentioned above are used as such device link profiles.
Meanwhile, in a printer using a four-color material of CMYK, i.e., using four-color toner or ink, for example, even if inputted color data CD1 request “yellow (Y)”, a small amount of color materials such as MYK usually happens to mix in a color material in addition to yellow. The same applies to cases of cyan and magenta.
Thus, in many cases, a small amount of other colors mixes even if a pure color is required. On this occasion, when an image to be printed is a photo image, no problems arise. However, when an image to be printed is a colored character or a solid color that is used in document-processing software, a small amount of mixed color material is conspicuous and the color becomes muddy, causing the image to be undesirable.
Accordingly, when a pure color such as “yellow”, “cyan” or “magenta” is represented, mix of small amount of other color materials is eliminated and each of the pure colors is expressed by only one color material. In addition, when a pure color such as “red”, “green” or “blue” is represented, the color is expressed by only a combination of two colors of “yellow” and “magenta”, of “yellow” and “cyan” or of “magenta” and “cyan”, respectively, and mix of small amount of other color materials is eliminated. In this way, a process is performed of representing color data using only one or two color materials, which is herein referred to as color purification or a color purification process.
According to a conventional color purification method described in Japanese unexamined patent publication No. 2002-152536, hue shift is performed on color data (a color signal) on an input side in a device-independent color space. Since the shift amount differs for each hue, a hue is shifted by an amount according to a hue of each piece of color data on the input side.
The conventional method mentioned above, however, assumes to use a certain particular input profile. Accordingly, the use of the above-described method by any other input profiles causes outputs to be unnatural.
When an output device is a printer, for example, a difference between a refined color and the surrounding colors that are not refined stands out in an image. As a difference between color spaces is larger, the color difference is greater, which increases the unnaturalness of outputs.
As a measure therefor, it is conceivable to understand differences between respective hues in color spaces of input devices and to prepare output profiles corresponding to respective input profiles. Stated differently, printer profiles are created and kept for one printer, the number of printer profiles being equal to the number of profiles of expected input devices.
In such a method, however, a color management module keeps many profiles for input devices, causing problems of requiring a considerable labor for creating the profiles and of requiring a large memory area for storing the profiles.